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View Full Version : 12 degree timing advance equals 400 RPM's



smbarcelow
09-04-2007, 08:23 AM
Well, I finally put a timing light on the 350 in our new (to us) Ski Sporter and found it 4 degrees retarded. Baseline timing should be 8 degrees advanced. This took care of the issue of all that backfiring through the carb and missing. It also took top end RPM's from 4500 to 4900 (flirting with 5 grand). She's running MUCH better. I think I have some fuel delivery issues to work through yet, but they're minor. Anyway, we had a good weekend of boating with our new toy.:cool:

MOP
09-04-2007, 11:33 AM
8 degrees is pretty conservative, more important is top timing. For top timing and good performance you should be a tad over 30 degrees, most run 32 to 34. For a dead stock setup 30-32 is decent.

Phil

justleft
09-04-2007, 12:44 PM
A total advance of 32-34 will require premium gas.

MOP
09-04-2007, 01:53 PM
A total advance of 32-34 will require premium gas.

Not so! I am running a 383 stroker with 9:125 at 32 degrees it runs great on 87, I just upped it to 34 degrees and went to 89 only due to the fact it had mostly winter gas in the tank.

Phil

smbarcelow
09-04-2007, 04:14 PM
I plan to run premium all the time, so that's not a problem. I'm guessing compression is somewhere around 9:1 but I'm not sure. It has aluminum heads, so they're probably not original. (Somebody correct me if I'm wrong about that. I assume most of the OE GM 350's had cast iron.) I haven't had an opportunity to check full advance yet. A mechanic friend of mine suggested that I target around 32 or 33 degrees, as well. I'm just getting started figuring this boat out, but I was real happy to find a no-brainer to fix first.

Pismo
09-04-2007, 07:13 PM
I plan to run premium all the time, so that's not a problem. I'm guessing compression is somewhere around 9:1 but I'm not sure. It has aluminum heads, so they're probably not original. (Somebody correct me if I'm wrong about that. I assume most of the OE GM 350's had cast iron.) I haven't had an opportunity to check full advance yet. A mechanic friend of mine suggested that I target around 32 or 33 degrees, as well. I'm just getting started figuring this boat out, but I was real happy to find a no-brainer to fix first.

You can run plain old 87/89 and get a tad more power.

MOP
09-04-2007, 07:25 PM
High test will slow you down unless you have the compression to support it, fuel needs to match the engine setup. I would not bother with HT unless you were 9.5 or better, many of our members have had better performance all the way around on regular.

Phil

VetteLT193
09-05-2007, 07:18 AM
High test will slow you down unless you have the compression to support it, fuel needs to match the engine setup. I would not bother with HT unless you were 9.5 or better, many of our members have had better performance all the way around on regular.
Phil

I have to disagree: There is no reason why premium will slow you down / produce less power, etc.

I do agree that there is no reason to spend the money on it unless you have the compression ratio to support it, but it isn't going to hurt anything if you want to run it for whatever reason.

gcarter
09-05-2007, 07:46 AM
I have to disagree: There is no reason why premium will slow you down / produce less power, etc.
I do agree that there is no reason to spend the money on it unless you have the compression ratio to support it, but it isn't going to hurt anything if you want to run it for whatever reason.
You know, taking a poll here won't change chemistry........
the lower the octane #, the more energy there is. Forty to fifty octane has even more energy, but you wouldn't like the ignition characteristics. High octane #'s only allow you to use a higher c/r w/o detonation.

smbarcelow
09-05-2007, 08:29 AM
I agree that 92 octane gasoline is slightly less reactive and, therefore, produces slightly less energy than 87 octane gasoline. Since I don't know for sure what my compression is, I thought it was cheap insurance. As I get to know more about the setup I have, I'll reconsider my fuel choice. (I suppose I could always run a tank of 87 and listen carefully for detonation.)

On the topic of fuel delivery, I found out last night that one of my accelerator pumps (Holley double-pumper) is frozen up. I was out there to play, so I haven't taken the carb apart to find out why yet. This was another nice find, as it explains the engine falling on its face when I punch the throttle.

VetteLT193
09-05-2007, 08:42 AM
You know, taking a poll here won't change chemistry........
the lower the octane #, the more energy there is. Forty to fifty octane has even more energy, but you wouldn't like the ignition characteristics. High octane #'s only allow you to use a higher c/r w/o detonation.

The energy is based on the number of carbon bonds within the fuel, not the octane. It is possible to have less energy with a higher octane, and maybe some time ago premium fuel carried a lower overall energy, but this isn't the case today by everything I understand of modern day gas formulations.

blackhawk
09-05-2007, 09:20 AM
Higher octane burns slower and has a lower flashpoint. That is why it is used in high compression motors with advanced timing. Without it you would experience detination. Now, using more octane that your motor needs will create less heat and heat=hp. So higher octane than needed will lower hp. If you don't believe me go put 118 octane in your boat and see how it runs.

Now, the difference between the readily available fuels(87,89,92) isn't going to make a big difference. In fact, it probably won't even be noticeable.

Rootsy
09-05-2007, 04:33 PM
engine geometry and dynamics are going to dictate what you run for full ignition timing in order to make maximum power / torque more so than fuel, especially in the pump versions. If that timing amount is too much for the specific fuel then you either have to change fuel, detune the engine or modify the combination to be more appropriate for the fuel you wish to utilize. we aren't talking about VP's, ERC's, etc etc etc that will make ice on a carburetor on an august day in Death Valley....

Cuda
09-05-2007, 06:14 PM
You should not have any problem with detonation at 32* with your CR using 87. As to whether it make more or less horsepower on a higher octane, I'm with gcarter on this issue, although I'm certainly not a petrochemical engineer, but I guarantee you can't tell the difference in speed, only in your wallet. That's providing there is no detonation. Detonation can also be caused by a piece of carbon build up that makes it ignite too soon, or a sharp burr on your piston, or other various things, but, imho, it won't be caused by running 87 octane in that engine.

BigGrizzly
09-05-2007, 06:27 PM
Actually Rootsey is correct and everybody is right and wrong. Higher octane on todays fuel and engine combinations in a engine will produce less power if the compression and timing are not sufficient to use it ! The reason being ALL the caloric heat in the fuel is not utilized therefor the fuel leaves the engine not completely used. What will be noticed much more than power loss but fuel economy. I have spent an awful lot of time At my old job and my hoby finding this out. My Criterion usually gets better fuel economy than a stock 502, because of this education. no one ever smells fuel from my exhaust, and my transom is not sooty. As much fuel is used as possible. Also using a lower grade of fuel than the engine can use will result in the same out come. If you doubt me that is OK, unless you have done the study with a dyno and fuel ratios your just guessing.Been there done that!!! It seems that this comes up every three months.

smbarcelow
09-05-2007, 09:39 PM
Yeah, I've read similar posts on The Formula Source forums from time to time. (My other toy is a 94 Firebird with an LT1.) I've actually had some practical experience with this. I had a 96 GMC Sonoma with a 4.3 liter that spark knocked like crazy unless I ran premium. I figured the additional cost of 92 was offset by better fuel economy due to less knock sensor interference. Anyway, point taken regarding unnecessary use of premium. Like I said, until I know what I have, it's cheap insurance.

BigGrizzly
09-06-2007, 09:23 AM
One thing I did neglect to say is if in doubt go for the higher octain.

Dredgeking
09-13-2007, 09:45 AM
Looks like you bought my old boat. I just sold it on ebay this summer. Looks like the buyers didn't keep it too long. It has a Chevy ZZ4 crate engine in it. You can find specs at Jegs or Summit on the ZZ4. Hope you enjoy it.

smbarcelow
09-17-2007, 11:28 AM
We're enjoying it very much! Thanks for posting the info on the engine. It's sure to come in very handy.